World powers on Saturday will seek to narrow their differences on Syria when they begin a second round of talks on establishing the framework for a political resolution of that country's prolonged and costly civil war.

The talks in Vienna are expected to focus on identifying Syria’s moderate opposition groups, as well as those that are considered terrorist entities.

The United States and its allies may work on disagreements with Russia and Iran about the political future of Syrian President Bashar al-Assad. They accomplished little in the previous round of meetings last month.

"[Russian Foreign Minister] Sergei Lavrov and [Iranian] Foreign Minister [Mohammad Javad] Zarif and I and others agreed to disagree,” Kerry said as the earlier talks wrapped up October 30.

Russia and Iran support the Assad regime. The U.S. says the Syrian leader must step down if there is to be any political resolution aimed at ending the war.

Russian President Vladimir Putin weighed in on the issue Friday, saying the Syrians alone would have to decide on Assad’s future. His comments were reported by Russian and Turkish news agencies.

After Kerry arrived in Vienna late Friday, he held bilateral talks with the U.N. envoy for Syria, Staffan de Mistura, and foreign ministers from Turkey and Saudi Arabia.

Asked whether the world powers could agree on which Syrian opposition groups could join the political transition talks, Saudi Foreign Minister Adel al-Jubeir smiled and said, “We will find out tomorrow.”

Notably absent from the second round of talks is Iran's Zarif. Iranian state media said Deputy Foreign Minister Hossein Amir-Abdollahian would attend, “not as a spectator” but “as a serious participant.” Tehran did not explain Zarif’s absence.

The State Department said 16 nations would be taking part in the talks, along with representatives of the United Nations, the European Union and the Arab League.

Less Than Optimistic

Before he left Washington, Kerry's last public comment on the prospects for the Vienna talks were less than optimistic. The secretary of state said he was unable to state that the world powers meeting in Austria were “on the threshold of a comprehensive agreement” about Syria.

“There remains a lot of work to be done,” he said.

Analysts voiced hesitant, sometimes pessimistic views about the chance of progress in Vienna.

“So this meeting is going to accomplish exactly what?” asked Anthony Cordesman, a defense and security scholar at the Center for Strategic and International Studies.

Even if world powers can reach an accord, Cordesman said, “what legitimacy would that agreement have in terms of the views of these [opposition] factions within Syria?”

Nevertheless, world powers are motivated to make progress, said Perry Cammack, a Middle East analyst at the Carnegie Endowment for International Peace.

“The fact that both Washington and Moscow are anxious for an agreement,” he said, “gives us some basis at least for discussions.”

However, Cammack said he did not expect the negotiators' hope for agreement to translate into quick political results in Damascus.

Kerry traveled to Vienna from Tunis, his first stop after leaving Washington on Thursday. The top U.S. diplomat and his aides took part in strategic dialogue with Tunisian officials.

After Vienna, Kerry is due to travel to Turkey, where he will join President Barack Obama at a summit of the world's biggest economic powers, the G-20 nations.